1 doctor weighed in:

Why does your body shape permanently change after you have a child?

1 doctor weighed in

High levels of Estrogens cause growth of the breast, selective growth of some (not all) fat tissues, and selective bone growth during pregnancy; fat may disappear later, but the other changes are irreversible.

In brief:
Hormones:

High levels of Estrogens cause growth of the breast, selective growth of some (not all) fat tissues, and selective bone growth during pregnancy; fat may disappear later, but the other changes are irreversible.
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